The high speed and long range of modern airborne vehicles places increasing range demands on radar systems used for tracking. The long-range requirement also requires the use of relatively high transmitted power to reliably detect small targets. High transmitted power implies a relatively higher peak transmitter power or a longer duration transmitter pulse (also known as a "wider" pulse). Assuming a maximum available peak power, longer range implies a longer duration transmitted pulse. A longer duration pulse tends to reduce range resolution, which is the ability to distinguish among targets which are at similar ranges. Pulse compression techniques can be used to improve range resolution in spite of the longer pulse duration, thus eliminating the need for high peak power short pulses, but in a monostatic radar, in which antenna and portions of the RF section are used both in transmission and reception, the minimum range at which a target can be detected increases with the transmitted pulse length. Thus, if the transmitter pulse duration is 100 microseconds (.mu.s), the minimum distance at which a target may be detected is about 8 nautical miles (nm). Clearly, a monostatic radar using pulses of such a duration cannot be used to detect close-in targets, as for example aircraft which are landing or taking off from an airport at which the radar is situated. An additional problem associated with pulse compression is the appearance of range sidelobes (as distinguished from antenna sidelobes) in addition to the main range lobe. The time position, or range, of the main lobe is the position that is tested for the presence of a target and for estimating the parameters of that target (reflected energy or power, closing speed, fluctuations in echo power and closing speed, etc.). The presence of range sidelobes on the compressed pulse results in interfering echoes which originate at ranges other than the range of the main lobe. This interference, known as "flooding," can cause erroneous estimates of the echo characteristics in the range cell (i.e., range increment) covered by the main lobe. Prior art techniques for suppressing range sidelobes include the "zero-Doppler" technique, in which the range sidelobe suppression scheme is based in part upon the assumption that the interfering echoes, as well as the desired echo, have a closing velocity that has no significant Doppler phase change or shift over the duration of the uncompressed pulse. The Doppler phase shift .phi..sub.DV across the uncompressed pulse is 2.pi. times the product of the Doppler frequency shift and the uncompressed pulse duration (i.e. .phi..sub.DV =2.pi. f.sub.d T.sub.0 radians). When this Doppler phase shift is actually zero or very small, moderate sidelobe suppression is achievable with the zero Doppler design. However, the zero Doppler design is very sensitive to small Doppler frequency shifts, making deep sidelobe suppression impossible for radar applications in which very deep sidelobe suppression is desired, as for example in weather mapping, clear air turbulence detection, and microburst detection.
U.S. Pat. No. 5,173,706, issued Dec. 22, 1992 in the name of Urkowitz, describes a pulse radar system in which Doppler processing is used to separate returns into frequency bins representative of radial speed. Interference from scatterers at other ranges is reduced by range sidelobe suppression filtering applied to the signal in each frequency bin.
U.S. Pat. No. 5,151,702, issued Sep. 29, 1992 in the name of Urkowitz, hereby incorporated by reference, describes a scheme for reducing range sidelobes by modulating sequentially transmitted radar pulses with first and second phase codes, which are selected so that, after separate pulse compression and matched filtering, the resulting compressed pulses each include main lobes which represent the range of the target, and also include range sidelobes which may introduce range ambiguity which range lobes are of mutually opposite sign. The compressed pulses are brought into time alignment and summed, whereby the main range lobes add and the range sidelobes cancel. The resulting range signal is then further processed for display.
The rapidity of modern transport systems requires radar systems with relatively rapid response times. For example, U.S. Pat. No. 5,103,233, issued Apr. 7, 1992 in the name of Gallagher et al., describes a radar system in which great attention is given to rapid operation in order to reduce the interval between scans of particular regions in an air traffic control context. The Gallagher et al. patent also describes a scheme for improving range resolution in the presence of clutter for detection of weather phenomena, separately claimed in U.S. Pat. No. 5,173,706, issued Dec. 22, 1992 in the name of Urkowitz. The improvement or range resolution in the presence of clutter is also important in other contexts, for example the detection and tracking of incoming missiles having very small radar cross-sections, in the presence of clutter such as sea action. In such a context, range of detection may be among the most important characteristics, because of the very short time which the speeds of such missiles allow for countermeasures. Improved methods and apparatus for reducing range sidelobes are desirable.